Hydraulic hybrid propel circuit with hydrostatic option and method of operation

ABSTRACT

A method of propelling a vehicle with a hybrid mode and a hydrostatic mode includes determining if a current propulsion mode is hybrid and if a selected mode is hydrostatic. A first transition mode is entered if the selected mode is hydrostatic and the current mode is hybrid. An engine-pump displacement target is set in the first transition mode. The method may include determining if the current mode is hybrid, hydrostatic, or a no-propulsion mode and if the selected mode is hybrid, hydrostatic, or no-propulsion. The engine-pump displacement target may be matched to a system consumption and an accumulator isolation valve closed when an engine-pump output matches the system consumption in the first transition mode. The method may include entering a second transition mode if the selected mode is hybrid and the current mode is hydrostatic. A method of configuring a propulsion mode from hybrid to hydrostatic includes configuring a drive motor displacement target to full displacement, matching a pump displacement to system consumption, and closing an accumulator isolation valve when a pump flow output matches the system consumption.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a U.S. National Stage of PCT/US2015/029366, filed on6 May 2015, which claims benefit of U.S. Patent Application Ser. No.61/989,335 filed on May 6, 2014, and which applications are incorporatedherein by reference. To the extent appropriate, a claim of priority ismade to each of the above disclosed applications.

BACKGROUND

Work machines can be used to move material, such as pallets, dirt,and/or debris. Examples of work machines include fork lifts, wheelloaders, track loaders, excavators, backhoes, bull dozers, telehandlers,etc. The work machines typically include a work implement (e.g., a fork)connected to the work machine. The work implements attached to the workmachines are typically powered by a hydraulic system. The hydraulicsystem can include a hydraulic pump that is powered by a prime mover,such as a diesel engine. The hydraulic pump can be connected tohydraulic actuators by a set of valves to control flow of pressurizedhydraulic fluid to the hydraulic actuators. The pressurized hydraulicfluid causes the hydraulic actuators to extend, retract, or rotate andthereby cause the work implement to move.

The work machine may further include a propulsion system adapted topropel the work machine. The propulsion system may include a hydraulicpump that is powered by the prime mover. The propulsion system mayinclude a hydrostatic drive.

SUMMARY

One aspect of the present disclosure relates to a method of propelling amobile work vehicle with a hybrid propulsion mode and a hydrostaticpropulsion mode. The method includes: 1) determining if a currentpropulsion mode is the hybrid propulsion mode; 2) determining if aselected propulsion mode is the hydrostatic propulsion mode; 3) enteringa first transition mode from the hybrid propulsion mode if the selectedpropulsion mode is the hydrostatic propulsion mode and the currentpropulsion mode is the hybrid propulsion mode; and 4) setting anengine-pump displacement target when in the first transition mode. Incertain embodiments, the method may include determining if the currentpropulsion mode is the hybrid propulsion mode, the hydrostaticpropulsion mode, or a no-propulsion mode; and determining if theselected propulsion mode is the hybrid propulsion mode, the hydrostaticpropulsion mode, or the no-propulsion mode. The method may furtherinclude substantially matching the engine-pump displacement target to asystem flow consumption, when in the first transition mode, and closingan accumulator isolation valve when an engine-pump flow output matchesthe system flow consumption, when in the first transition mode. Themethod may further include closing the accumulator isolation valve whenboth the engine-pump flow output matches the system flow consumption anda drive-motor pressure rate of change is greater than a predeterminedvalue when in the first transition mode. The method may further includeentering a second transition mode from the hydrostatic propulsion modeif the selected propulsion mode is the hybrid propulsion mode and thecurrent propulsion mode is the hydrostatic propulsion mode.

Another aspect of the present disclosure relates to a method ofconfiguring a propulsion mode of a mobile work vehicle from a hybridpropulsion mode to a hydrostatic propulsion mode. The methodincludes: 1) determining if a selected propulsion mode is thehydrostatic propulsion mode; 2) configuring a drive motor displacementtarget of a drive motor to full displacement if the selected propulsionmode is the hydrostatic propulsion mode; 3) substantially matching anengine-pump displacement target to a system flow consumption if theselected propulsion mode is the hydrostatic propulsion mode; and 4)closing an accumulator isolation valve when an engine-pump flow outputmatches the system flow consumption and the selected propulsion mode isthe hydrostatic propulsion mode.

A variety of additional aspects will be set forth in the descriptionthat follows. These aspects can relate to individual features and tocombinations of features. It is to be understood that both the foregoinggeneral description and the following detailed description are exemplaryand explanatory only and are not restrictive of the broad concepts uponwhich the embodiments disclosed herein are based.

DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments are described with referenceto the following figures, which are not necessarily drawn to scale,wherein like reference numerals refer to like parts throughout thevarious views unless otherwise specified.

FIG. 1 is a schematic diagram of a hydraulic system having features thatare examples according to the principles of the present disclosure;

FIG. 2 is a schematic diagram of the hydraulic system of FIG. 1 furtherillustrating a control system of the hydraulic system;

FIG. 3 is the schematic diagram of FIG. 1 further illustrating a firstmode of the hydraulic system:

FIG. 4 is the schematic diagram of FIG. 1 further illustrating a secondmode of the hydraulic system;

FIG. 5 is the schematic diagram of FIG. 1 further illustrating a thirdmode of the hydraulic system;

FIG. 6 is the schematic diagram of FIG. 1 further illustrating a fourthmode of the hydraulic system;

FIG. 7 is the schematic diagram of FIG. 1 further illustrating a fifthmode of the hydraulic system;

FIG. 8 is a schematic diagram of another hydraulic system havingfeatures that are examples according to the principles of the presentdisclosure;

FIG. 9 is a schematic top plan view of a work vehicle including thehydraulic system of FIG. 1 or 8 according to the principles of thepresent disclosure:

FIG. 10 is a schematic diagram of still another hydraulic system havingfeatures that are examples according to the principles of the presentdisclosure;

FIG. 11 is a state chart of a transmission mode supervisory controlsystem according to the principles of the present disclosure, the statechart including a hybrid mode, a hydrostatic mode, and two transitionmodes between the hybrid mode and the hydrostatic mode;

FIG. 12 is a supervisory flow chart including a transmission modeprocess, a drive motor supervisory process, an engine and pumpsupervisory process, and a valve supervisory process according to theprinciples of the present disclosure;

FIG. 13 is a transmission mode flow chart suitable for use in thetransmission mode process of FIG. 12;

FIG. 14 is a drive motor supervisory flow chart suitable for use withthe drive motor supervisory process of FIG. 12;

FIG. 15 is an engine and pump supervisory flow chart suitable for usewith the engine and pump supervisory process of FIG. 12; and

FIG. 16 is a valve supervisory flow chart suitable for use with thevalve supervisory process of FIG. 12.

DETAILED DESCRIPTION

Various embodiments will be described in detail with reference to thedrawings, wherein like reference numerals represent like parts andassemblies throughout the several views. Reference to variousembodiments does not limit the scope of the claims attached hereto.Additionally, any examples set forth in this specification are notintended to be limiting and merely set forth some of the many possibleembodiments for the appended claims.

The present disclosure relates generally to hydraulic circuitarchitectures for use in work vehicles. A hydraulic circuitarchitecture, in accordance with the principles of the presentdisclosure, can include a propel circuit and a work circuit. In certainembodiments, the propel circuit and the work circuit can be powered by asame hydraulic pump structure (e.g., a hydraulic pump or a hydraulicpump/motor). In certain embodiments, the hydraulic pump structureincludes a single drive pump (e.g., only one pump, only one pumpingrotating group, only one pump/motor, etc.). In certain embodiments, thepropel circuit can include a hydraulic accumulator and a hydraulicpropulsion pump/motor for powering propulsion elements (e.g., wheels,tracks, etc.) of the work vehicle through a drivetrain. The work circuitcan include various actuators for powering work components such aslifts, clamps, booms, buckets, blades, and/or other structures. Thevarious actuators may include hydraulic cylinders, hydraulic motors,etc. In a preferred embodiment, the hydraulic architecture is used on aforklift 50 (see FIG. 9) where the propulsion circuit powers adrivetrain 114 coupled to drive wheels 54 of the forklift 50, and thework circuit includes valving and actuators (e.g., hydraulic cylinders)for raising and lowering a fork 52 of the forklift 50, for front-to-backtilting of the fork 52, and for left and right shifting of the fork 52.

In certain embodiments, the hydraulic accumulator of the propulsioncircuit can be used to provide numerous functions and benefits. Forexample, the provision of the hydraulic accumulator allows the hydraulicpump/motor and prime mover powering the propulsion circuit to beconsistently operated at peak efficiency or near peak efficiency.Moreover, accumulated energy in the hydraulic accumulator can be used toprovide power for starting a power source (e.g., a prime mover, a dieselengine, or other engine) used to drive the hydraulic pump/motor.Additionally, the hydraulic accumulator can be used to providepropulsion functionality even when the power source coupled to thehydraulic pump/motor is not operating. Similarly, the hydraulicaccumulator can be used to provide work circuit functionality even whenthe power source coupled to the hydraulic pump/motor is not operating.Furthermore, by operating the propulsion hydraulic pump/motor as a motorduring braking/deceleration events, energy corresponding to thedeceleration of the work vehicle can be back-fed and stored by thehydraulic accumulator for later re-use to enhance overall efficiency ofthe work vehicle.

In certain embodiments, one (i.e., a single) hydraulic pump/motor (e.g.,a hydraulic pump/motor 102, shown at FIG. 1) is used to power both thepropulsion circuit and the working circuit. In such an embodiment, acircuit selector (i.e., a mode selector) can be provided for selectivelyplacing a high pressure side of the hydraulic pump/motor in fluidcommunication with either the propulsion circuit or the working circuit.The circuit selector can include one or more valves. Furthermore, across-over valve can be provided for selectively providing fluidcommunication between the propulsion circuit and the work circuit. Byopening the cross-over valve, power from the hydraulic accumulator canbe used to drive one or more actuators of the work circuit therebyallowing for actuation of the actuators of the work circuit, even whenthe power source is turned off. When the circuit selector has placed thepump/motor in fluid communication with the propulsion circuit forpropelling the work vehicle, the various components of the work circuitcan be actuated by opening the cross-over valve. Additionally, when thecircuit selector has placed the pump/motor in fluid communication withthe work circuit, the hydraulic accumulator can be used to provide forpropulsion and steering of the work vehicle. It will be appreciated thata steering component is preferably incorporated into the hydraulicpropulsion circuit. When the power source is turned off, the hydraulicaccumulator can be used to power the steering component, power thepropulsion elements, and/or power the various components of the workcircuit. It will be appreciated that such activities can be conductedindividually or simultaneously. The cross-over valve can provide avariable size orifice.

In certain embodiments, the hydraulic pump/motor coupled to the powersource is an open circuit pump/motor having a rotating group and a swashplate that is adjustable to control an amount of hydraulic fluiddisplaced by the pump/motor per rotation of a pump/motor shaft by thepower source. In certain embodiments, the swash plate has an over-centerconfiguration. When the pump/motor is operating as a pump, the swashplate is on a first side of center and the power source rotates thepump/motor shaft in a first direction such that hydraulic fluid ispumped through the pump/motor from a low pressure side in fluidcommunication with a reservoir/tank to a high pressure side in fluidcommunication with the circuit selector. When the hydraulic pump/motoris operated as a motor, the swash plate may be moved to a second side ofcenter and hydraulic fluid from the hydraulic accumulator is directedthrough the pump/motor from the high pressure side to the low pressureside thereby causing the pump/motor shaft to rotate in the samerotational direction that the pump/motor shaft rotates when driven bythe power source. In this way, hydraulic energy from the hydraulicaccumulator can be used to start modes including use of the powersource.

The propulsion pump/motor can also be an open circuit pump/motor havinga low pressure side connected to the reservoir/tank and a high pressureside that connects to the hydraulic pump/motor coupled to the powersource through the circuit selector. The propulsion pump/motor caninclude a rotating group and a swash plate that can be adjusted tocontrol displacement of the propulsion pump/motor for each revolution ofa shaft of the propulsion pump/motor. The swash plate can be anover-center swash plate that allows for bi-directional rotation of theshaft of the propulsion pump/motor. For example, when the swash plate ison a first side of center, hydraulic fluid flow through the pump/motorfrom the high pressure side to the low pressure side can drive the shaftin a clockwise direction. In contrast, when the swash plate is on asecond side of center, hydraulic fluid flow through the propulsionpump/motor in a direction from the high pressure side to the lowpressure side causes rotation of the shaft in a counterclockwisedirection. In this way, the propulsion pump/motor can be used to drivethe work vehicle in both forward and rearward directions. Moreover,during a braking event, the propulsion pump/motor can function as a pumpand can direct hydraulic fluid from the reservoir to the hydraulicaccumulator to charge the hydraulic accumulator thereby capturing energyassociated with the deceleration. Thus, the propulsion pump/motor andthe hydraulic accumulator provide a braking/deceleration and energystorage function. It will be appreciated that in other embodiments(e.g., an embodiment illustrated at FIG. 8), valving can be used incombination with non-over-center pump/motors to provide the same orsimilar functionality as the over-center pump/motors described above.The non-over-center pump/motors and the valving can be used as thehydraulic pump/motor coupled to the power source, as shown at FIG. 8,and/or can be used as the propulsion hydraulic pump/motor that iscoupled to the drivetrain.

Further details of such a hydraulic circuit architecture are describedand illustrated at U.S. Patent Publication US 2013/0280111 A1 which ishereby incorporated by reference in its entirety. FIGS. 1-10 illustratevarious hydraulic circuits and a control system 500 and furtherillustrate the hydraulic circuit architecture in a context of a workmachine 50. Methods of operating such a hydraulic circuit architectureare described and illustrated hereinafter.

According to the principles of the present disclosure, methods ofoperating the hydraulic circuit architecture provide smooth andbeneficial use of the work machine 50. Hydraulic hybrid vehiclestypically operate at pressures below a maximum system operating pressureto allow for energy storage capacity in an accumulator and to increaseoperating displacement of a pump and motor to increase pump and motorefficiency. However, this typically limits the torque that can bedelivered quickly to a drivetrain when climbing a hill, acceleratinghard, or any other time that high torque is desired. This lack ofinstantaneous torque can be eliminated by isolating the high pressureaccumulator from the system and operating the vehicle in a typicalhydrostatic mode where pressure (and thereby torque) can be raised veryquickly and to pressure levels that may exceed an operating pressure ofthe high pressure accumulator.

Turning now to FIG. 11, an example transmission mode supervisory controlstate machine 650 is illustrated according to the principles of thepresent disclosure. As depicted, the control state machine 650 includesa hybrid mode 660, a hydrostatic mode 670, a first transition mode 680,and a second transition mode 690. The first transition mode 680 isactivated when transitioning from the hybrid mode 660 to the hydrostaticmode 670. Likewise, the second transition mode 690 is activated whentransitioning from the hydrostatic mode 670 to the hybrid mode 660. Asdepicted, a path 692 illustrates a switch from the hybrid mode 660 tothe first transition mode 680. Likewise, a path 694 illustratesswitching from the first transition mode 680 to the hydrostatic mode670. Similarly, a path 696 illustrates switching from the hydrostaticmode 670 to the second transition mode 690. And, a path 698 illustratesswitching from the second transition mode 690 to the hybrid mode 660. Asdepicted, the supervisory control state machine 650 includes twotransmission modes 660, 670 and two transitional modes 680, 690. Inother embodiments, additional modes, additional transition modes, and/oradditional paths between the various modes may be included.

A state of the transmission state machine 650 is determined by thecombination of a selected transmission mode 652 and a currenttransmission mode 654 and their respective values as determined by thelogic outlined in the flow charts 750A and 750B of FIG. 13. The hybridmode 660 of the control state machine 650 may include functional andoperational characteristics of and/or may activate a hybrid propel mode84, further described hereinafter. When the selected transmission mode652 is set to the hybrid propel mode 84, and the current transmissionmode 654 is set to the hybrid propel mode 84, the state of thetransmission state machine 650 is set to the hybrid propel mode 660. Thehydrostatic mode 670 likewise may include operational and functionalcharacteristics of and/or may activate a hydrostatic mode 86, furtherdescribed hereinafter. When the selected transmission mode 652 is set tothe hydrostatic mode 86, and the current transmission mode 654 is set tothe hydrostatic mode 86, the state of the transmission state machine 650is set to the hydrostatic propel mode 670.

As depicted at FIG. 11, the transmission mode supervisory control statemachine 650 (i.e., the supervisory controller) has the two states of thehybrid mode 660 and the hydrostatic mode 670. In other embodiments,additional states may be included. For example, a work circuit statethat includes operational and functional characteristics of and/oractivates a work circuit primary mode 82, described hereinafter, may beincluded.

The transition modes 680, 690 are defined to control transitionalbehavior between the states 660, 670. In particular, the firsttransition mode 680 controls the transitional behavior when switchingfrom the hybrid mode state 660 to the hydrostatic mode state 670.Likewise, the second transition mode 690 controls the transitionalbehavior when switching from the hydrostatic mode state 670 to thehybrid mode state 660. In other embodiments, other transitional modesmay be defined to and from the various other states (e.g. a stateincluding operational and functional characteristics of and/oractivating the work circuit primary mode 82).

The current transmission mode 654 is defined by the existing state ofthe valves and system actuators. The selected transmission mode 652 isdefined by operator behavior. The state of the transmission mode of thecontrol state machine 650 defines the hybrid system component behaviorwhen in the hybrid mode 660. Likewise, the state of the transmissionmode of the control state machine 650 defines the hydrostatic systemcomponent behavior when in the hydrostatic mode 670. When in the firsttransition mode 680, the selected transmission mode 652 is thehydrostatic mode 86, and the current transmission mode 654 is the hybridpropel mode 84. Likewise, when in the second transition mode 690, theselected transmission mode 652 is set to the hybrid propel mode 84, andthe current transmission mode 654 is set to the hydrostatic mode 86. Thestate machine 650 is executed on every computational loop of thesupervisory algorithm, in certain embodiments. In the depictedembodiment, the current transmission mode is determined first, and theselected transmission mode is determined second.

Turning now to FIG. 12, an example supervisory flow chart 700 isillustrated according to the principles of the present disclosure. Inparticular, the supervisory flow chart 700 includes a transmission modeprocess 750. As depicted, the transmission mode process 750 includes acurrent transmission mode process 750A and a selected transmission modeprocess 750B. A path 705 illustrates transferring from the currenttransmission mode process 750A to the selected transmission mode process750B. The supervisory flow chart 700 further includes a drive motorsupervisory process 850. A path 715 illustrates transferring from thetransmission mode process 750 to the drive motor supervisory process850. The supervisory flow chart 700 further includes an engine and pumpsupervisory process 900. A path 725 illustrates transferring from thedrive motor supervisory process 850 to the engine and pump supervisoryprocess 900. The supervisory flow chart 700 further includes a valvesupervisory process 950. As depicted, a path 735 illustratestransferring from the engine and pump supervisory process 900 to thevalve supervisory process 950. A path 745 also illustrates transferringfrom the valve supervisory process 950 to the transmission mode process750.

Turning now to FIG. 13, an example flow chart illustrating thetransmission mode process 750 is illustrated according to the principlesof the present disclosure. The transmission mode flow chart 750 includesa current transmission mode process 750A and a selected transmissionmode process 750B. The current transmission mode process 750A determinesthe current or existing state of the transmission. This determination isbased on the known valve and actuator states and/or the commanded valveand actuator states if some or all of the valve position sensors are notavailable. The current transmission mode 654 is determined by what modeis currently being executed on a work machine 50. The currenttransmission mode process 750A is thereby a process used to calculatethe current transmission mode 654. The selected transmission modeprocess 750B is a process used to select the next transmission modestate based on operator controlled parameters and the existing sensors,valves, and actuator states of the work machine 50. The transmissionmode flow chart 750 includes a plurality of tests and evaluations todetermine the current transmission mode 654 and the selectedtransmission mode 652.

A first test set 800 of the current transmission mode process 750Aincludes a test 802 to determine if the operator desires to accelerateor decelerate. The first test set 800 also includes a test 804 todetermine if an accumulator isolation valve 210 is energized (i.e.,open). The first test set 800 further includes a test 806 to determineif a previously selected transmission mode 652 p is the hybrid propelmode 84. Logical values of results of each of the tests 802, 804, and806 are ANDed at a logical AND 808. In particular, the output of eachtest 802, 804, and 806, etc. are all binary and combined at the AND gate808 using Boolean logic.

The transmission mode flow chart 750 further includes a second test set810. The second test set 810 includes a test 811 to determine if theoperator desires to accelerate the work machine 50. The second test set810 includes a test 812 to determine if a prime mover 104 (e.g., anengine) is “on” (i.e., running). The second test set 810 includes a test813 to determine if the accumulator isolation valve 210 is de-energized(i.e., closed). The second test set 810 includes a test 814 to determineif a work circuit valve 206 (i.e., an engine pump on valve) isde-energized (i.e., closed). The second test set 810 includes a test 815to determine if a main isolation valve 208 is de-energized (i.e., open).The second test set 810 includes a test 816 to determine if adisplacement of a drive motor 108 (e.g., a pump/motor) has reached amaximum displacement. The second test set 810 includes a test 817 todetermine if the previously selected transmission mode 652 p is thehydrostatic mode 86. And, the second test set 810 includes a test 818 todetermine if a hydrostatic mode enable variable has been set to“enabled”. Logical values of results of each of the tests 811-818 areANDed at a logical AND 819. In particular, the output of each test811-818, etc. are all binary and combined at the AND gate 819 usingBoolean logic.

The transmission mode flow chart 750 further includes a third set oftests 820. The third set of tests 820 includes a test 821 to determineif the current transmission mode 654 is the hybrid propel mode 84. Thethird test set 820 includes a test 822 to determine if a target pressureof the pump/motor 102 is greater than a hydrostatic entry pressure. Thetarget pressure refers to the desired pressure that pump/motor 108 andpump/motor 102 should be operating at in order to achieve the operatorcommands. The hydrostatic entry pressure is a calibration that thetarget pressure needs to exceed in order to prevent the system fromentering hydrostatic mode 86 at too low of a command. The hydrostaticentry pressure sets the minimum threshold of target pressure to enterthe hydrostatic mode 86. The third test set 820 includes a test 823 todetermine if the pressure target of the pump/motor 102 is greater than acurrent pressure of an accumulator 116. The third test set 820 includesa test 824 to determine if an accelerator pedal percentage of full scaleactivation is greater than a threshold percentage for requesting entryto the hydrostatic mode 86. The third test set 820 includes a test 825to determine if the hydrostatic mode 86 is enabled. The tests 818 and825 may be combined in certain embodiments. The third test set 820includes a test 826 to determine if a flow demand of a work circuit 300is less than a hydrostatic entry flow. The hydrostatic entry flow is acalibration or preset constant value that prevents hydrostatic modeentry if too much work circuit flow demand is present (e.g., if workcircuit flow demand exceeds a predetermined value). The third test set820 includes a test 827 to determine if a current speed of the workmachine 50 is less than a maximum hydrostatic entry speed. The thirdtest set 820 includes a test 828 to determine if vehicle hot shift isnot preventing entry to the hydrostatic mode 86. Hot shift is changingthe forward-neutral-reverse switch (i.e., the FNR switch) directionintent to the opposite of the current direction of travel. In otherwords, putting the work machine 50 in reverse while traveling forwardand vice-a-versa. And, the third test set 820 includes a test 829 todetermine if a conditional timer has expired. A conditional timer meansthat the conditional tests 822-827 must be true for a predetermined timebefore test 829 will become true. The test 829 prevents signal noisefrom making (i.e., causing) a switch to a new state. Logical values ofresults of each of the tests 821-829 are logically ANDed at a logicalAND 831. In particular, the output of each test 821-829, etc. are allbinary and combined at the AND gate 831 using Boolean logic.

The transmission mode flow chart 750 includes a fourth set of tests 833.The fourth set of tests 833 includes a first subset of tests 834 and asecond subset of tests 835. The first subset of tests 834 includes atest 836 to determine if the current transmission mode 654 is thehydrostatic mode 86. The first subset of tests 834 includes a test 837to determine if the pressure target of the pump/motor 108 is less thanthe current pressure of the accumulator 116. The target pressure refersto the desired pressure that pump/motor 108 and pump/motor 102 should beoperating at in order to achieve the operator commands. The first subsetof tests 834 includes a test 838 to determine if the accelerator pedalpercentage of full scale activation is less than a threshold hydrostaticexit percentage. The first subset of tests 834 includes a test 839 todetermine if the operator desires the work machine 50 to decelerate. Thefirst subset of tests 834 includes a test 840 to determine if theoperator desires the work machine 50 to be in neutral. The first subsetof tests 834 includes a test 841 to determine if a state of the primemover 104 is “off”. And, the first subset of tests 834 includes a test842 to determine if any faults are present in the control system 500.Logical values of results of each of the tests 836-842 are logicallyORed at a logical OR 846. In particular, the output of each test836-842, etc. are all binary and combined at the OR gate 846 usingBoolean logic.

The second subset of tests 835 of the fourth set of tests 833 includes atest 843 that determines if a conditional timer has expired. Aconditional timer means that the conditional tests 836-842 must be truefor a predetermined time before test 843 will become true. The test 843prevents signal noise from making (i.e., causing) a switch to a newstate.

Logical values of results of each of the logical OR 846 and the resultsof the second subset of tests 835 are logically ANDed at a logical AND848. In particular, the output of each of the OR gate 846 and the test843, etc. are all binary and combined at the AND gate 848 using Booleanlogic.

As illustrated at FIG. 13, the transmission mode flow chart 750 maystart at a start position 752. As illustrated at FIG. 12, the path 745is a portion of a loop of the supervisory flow chart 700. As illustratedat FIG. 13, the path 745 may begin at the start position 752 or may flowfrom a valve supervisory flow process 950. The path 745, in each case,brings control to a decision point 754 that determines if the AND outputof the logical AND gate 808 is true (e.g., is a Boolean “1”). If thelogical AND 808 is true, the current transmission mode 654 is the hybridpropel mode 84 and is registered as such at block 756. If the output ofthe logical AND gate 808 is not true (e.g., is a Boolean “0”), controladvances to a decision point 758 that determines if the logical AND 819is true. If the logical AND 819 is true, the current transmission mode654 is the hydrostatic mode 86 and is registered as such at block 760.If the logical AND 819 is not true, the current transmission mode 654remains as previously registered (i.e., does not change) at block 762.The results of either the block 756, the block 760, or the block 762 aretransmitted as control passes along the path 705 from the currenttransmission mode process 750A of the transmission mode flow chart 750to the selected transmission mode process 750B of the transmission modeflow chart 750.

The selected transmission mode process 750B of the transmission modeflow chart 750 receives the information from the current transmissionmode process 750A. The results of the current transmission mode process750A are carried along with results from the selected transmission modeprocess 750B. The path 705 brings control to a decision point 774 wherethe logical AND 831 is evaluated. If the logical AND 831 is true, theselected transmission mode 652 is the hydrostatic mode 86 and is set andregistered as such at block 776. If the logical AND 831 is not true,control advances to a decision point 778 where the logical AND 848 isevaluated. If the logical AND 848 is true, the selected transmissionmode 652 is the hybrid propel mode 84 and is set and registered as suchat block 780. If the logical AND 848 is not true, the selectedtransmission mode 652 remains as the previously selected transmissionmode 652 p and is registered as such at block 782. The results of thecurrent transmission mode process 750A and the selected transmissionmode process 750B are transferred to the drive motor supervisory flowchart 850 along the path 715.

Turning now to FIG. 14, an example flow chart illustrating the drivemotor supervisory process 850 is illustrated according to the principlesof the present disclosure. The drive motor supervisory flow chart 850begins at the selected transmission mode process 750B, and the path 715transfers control to a decision point 860 that determines whether theselected transmission mode 652 is the hydrostatic mode 86. If the resultis “yes”, control is transferred to block 870, and a displacement of thedrive motor 108 (i.e., the pump/motor) is set to 100%. If the selectedtransmission mode 652 is not the hydrostatic mode 86, control istransferred to block 880 where the displacement of the drive motor 108is set according to a normal hybrid drive motor displacement targetcalculation. The drive motor displacement target is released to anelectronic control unit 502 at step 890. Control then passes to theengine and pump supervisory process 900.

Turning now to FIG. 15, an example flow chart illustrating the engineand pump supervisory process 900 is illustrated according to theprinciples of the present disclosure. The engine and pump supervisoryflow chart 900 begins at the current transmission mode process 750A.Control is transferred to a decision point 902 where the currenttransmission mode 654 is queried to see if it is set to the hydrostaticmode 86. If the result is “yes”, control advances to block 912 where anengine state target is set to “on”. The engine state target value of“on” is stored and released to the control system at step 932. Controlthen passes to block 914 where hydrostatic flow and pressure targets ofthe pump/motor 102, in cooperation with the prime mover 104, arecalculated. Control then transfers to block 916 where a hydrostatic modeengine speed target is calculated. The engine speed target is stored andreleased to the control system at step 936. Control then passes to block918 where a hydrostatic mode engine pump displacement target iscalculated. The resulting engine pump displacement target is stored andreleased to the system at step 938. If the result of decision point 902is “no”, control is transferred to block 922 where a hybrid mode enginestate target is calculated. The resulting engine state target is storedand released to the control system at the step 932. Control is thentransferred to block 924 where hybrid mode flow and pressure targets ofthe pump/motor 102, in cooperation with the prime mover 104, arecalculated. Control is then transferred to block 926 where a hybrid modeengine speed target is calculated. The resulting engine speed target isstored and released to the control system at the step 936. Control isthen transferred to block 928 where a hybrid mode engine pumpdisplacement target is calculated. The resulting engine pumpdisplacement target is stored and released to the control system at thestep 938. Upon the engine state target, the engine speed target, and theengine pump displacement target being calculated, control is passed tothe valve supervisory process 950.

Turning now to FIG. 16, an example flow chart illustrating the valvesupervisory process 950 is illustrated according to the principles ofthe present disclosure. The valve supervisory flow chart 950 begins atthe selected transmission mode process 750B. The valve supervisory flowchart 950 includes a first set of tests 980 and a second set of tests990. The first set of tests 980 include a test 981 that determines ifthe accumulator isolation valve 210 is energized (i.e., is open). Theset of tests 980 includes a test 982 that determines if the selectedtransmission mode 652 is the hydrostatic mode 86. The set of tests 980includes a test 983 that determines if a drive motor pressure rate ofchange is greater than zero. A pressure rate of change greater than zeroindicates that the engine pump is providing more flow into the systemthan the motor, valves, and other hydraulic components are consuming. Ifthe valve closes when this value is negative, the system may cavitate.The set of tests 980 includes a test 984 that determines if the enginespeed target is greater than a minimum hydrostatic speed for the engine(i.e., the prime mover 104). The set of tests 980 includes a test 985that determines if the engine speed status is greater than a minimumhydrostatic engine speed. And, the set of tests 980 includes a test 986that determines if a hydraulic pressure at the drive motor 108 (i.e.,the pump/motor) is greater than a current pressure at the hydraulicaccumulator 116. Logical values of results of each of the tests 981-986are ANDed and stored at a logical AND 987. In particular, the output ofeach test 981-986, etc. are all binary and combined at the AND gate 987using Boolean logic.

The second set of tests 990 includes a first test 991 that determines ifthe accumulator isolation valve 210 is de-energized (i.e., closed). Thesecond set of tests 990 includes a test 992 that determines if thecurrent pressure at the drive motor 108 is below a minimum hydrostaticmode entry pressure. And, the second set of tests 990 includes a test993 that determines if the prime mover 104 (e.g., the engine) is “off”.Logical values of results of each of the tests 991-993 are ORed andstored at a logical OR 994. In particular, the output of each test991-993, etc. are all binary and combined at the OR gate 994 usingBoolean logic.

Upon control entering the valve supervisory flow chart 950, a decisionpoint 952 evaluates whether the logical AND 987 is true. If the logicalAND 987 is true, control passes to block 954 where the accumulatorisolation valve 210 is closed (i.e., de-energize). If the logical valueof the AND 987 is not true, control passes to decision point 956 wherethe logical OR 994 is evaluated. If the logical value of the OR 994 is“true”, control passes to block 958 where the accumulator isolationvalve 210 is opened (i.e., energized). If the logical value of the OR994 is not true, control is transferred to block 960 where a currentstate of the accumulator isolation valve 210 is maintained. Upon thevalve supervisory flow chart 950 being completed, control is passedalong path 745 to the transmission mode process 750.

According to the principles of the present disclosure and as illustratedat FIGS. 1-7, a hydraulic system 100 (i.e., a hydraulic circuitarchitecture) is adapted to power the drivetrain 114 of the work machine50 (i.e., a work vehicle, a mobile work vehicle, a forklift, a lifttruck, a fork truck, a wheel loader, a digger, an excavator, a backhoeloader, etc.). The hydraulic system 100 may be further adapted to powera work circuit 300 of the work machine 50. The hydraulic system 100 maybe adapted to power a steering control unit 600 (e.g., a hydraulicsteering circuit) of the work machine 50. As depicted at FIG. 9, thework machine 50 includes a work attachment 52 (e.g., the fork, a workcomponent, etc.), at least one drive wheel 54, and at least one steerwheel 56. In certain embodiments, one or more drive wheel 54 may becombined with one or more steer wheel 56. In certain embodiments, thework machine 50 may include only a single drive hydraulic pump.

The hydraulic system 100 is adapted to recover energy and store theenergy in a hydraulic accumulator 116 for reuse. For example, when thework machine 50 is decelerated, the drivetrain 114 may deliver kineticenergy to the hydraulic system 100 and thereby store the energy in thehydraulic accumulator 116. The hydraulic system 100 is also adapted toquickly start a prime mover 104 (e.g., the internal combustion engine)of the work machine 50 with the energy stored in the hydraulicaccumulator 116. The hydraulic system 100 may be adapted to power thedrivetrain 114, the work circuit 300, and/or the steering control unit600 without the prime mover 104 running by drawing hydraulic power fromthe hydraulic accumulator 116. In certain embodiments, the prime mover104 may drive only a single hydraulic pump. In certain embodiments, theprime mover 104 may drive only a single hydraulic pump that powers thedrivetrain 114 and the work circuit 300. In certain embodiments, theprime mover 104 may drive only a single hydraulic pump that powers atleast the drivetrain 114 and the work circuit 300. In certainembodiments, the prime mover 104 may drive only a single hydraulic pumpthat powers the drivetrain 114, the work circuit 300, and the steeringcontrol unit 600. In certain embodiments, the prime mover 104 may driveonly a single hydraulic pump that at least powers the drivetrain 114,the work circuit 300, and the steering control unit 600.

The hydraulic system 100 operates in various modes depending on demandsplaced on the work machine 50 (e.g., by an operator). A control system500 monitors an operator interface 506 of the work machine 50 and alsomonitors various sensors 510 and operating parameters of the hydraulicsystem 100. As illustrated at FIG. 2, signal lines 508 may facilitatecommunication within the control system 500. The control system 500evaluates input received from the operator interface 506. In certainembodiments, an electronic control unit 502 monitors the various sensors510 and operating parameters of the hydraulic system 100 to configurethe hydraulic system 100 into the most appropriate mode. The modesinclude a work circuit primary mode 82, as illustrated at FIG. 3; thehybrid propel mode 84, as illustrated at FIGS. 4 and 5, and ahydrostatic mode 86, as illustrated at FIGS. 6 and 7. The electroniccontrol unit 502 may monitor the operator interface 506, the prime mover104, and environmental conditions (e.g. ambient temperature). Memory 504(e.g., RAM memory) may be used within the electronic control unit 502 tostore executable code, the operating parameters, the input from theoperator interface, etc.

In the work circuit primary mode 82 (see FIG. 3), power from the primemover 104 is directly supplied to the work circuit 300 by the hydraulicsystem 100, and power from the hydraulic accumulator 116 is delivered tothe drivetrain 114 by the hydraulic system 100. In certain embodiments,power for the steering control unit 600 is also taken from the hydraulicaccumulator 116 in the work circuit primary mode 82. The work circuitprimary mode 82 may be selected when power demands by the drivetrain 114are low, relatively low, and/or are anticipated to be low, and powerdemands and/or hydraulic flow demands by the work circuit 300 are high,relatively high, and/or are anticipated to be high. Such conditions mayoccur, for example, when the work machine 50 is moving slowly or isstationary and the work attachment 52 is being used extensively and/orwith high loading. In the work circuit primary mode 82, the steeringcontrol unit 600 may receive power from the hydraulic accumulator 116.

The hybrid propel mode 84 (see FIGS. 4 and 5) may be used when the powerdemand from the drivetrain 114 is dominate over the power demand of thework circuit 300. The hybrid propel mode 84 may also be used when it isdesired to recapture energy from the deceleration of the work machine50. The hybrid propel mode 84 may further be used to power the workmachine 50 without the prime mover 104 running or running full time. Forexample, the hybrid propel mode 84 allows the prime mover 104 to be shutdown upon sufficient pressure existing in the hydraulic accumulator 116.Upon depletion of the hydraulic accumulator 116 to a lower pressure, thehybrid propel mode 84 hydraulically restarts the prime mover 104 therebyrecharging the hydraulic accumulator 116 and also providing power to thework machine 50 from the prime mover 104. In the hybrid propel mode 84,the steering control unit 600 may receive power from the hydraulicaccumulator 116 and/or the prime mover 104.

The hydrostatic mode 86 (see FIGS. 6 and 7) may be used when the demandsof the drivetrain 114 are high, relatively high, and/or are anticipatedto be high. For example, when the work machine 50 is driven at a highspeed, when the work machine 50 is driven up an incline, and/or when thedrivetrain 114 is under a high load. The hydrostatic mode 86 may be usedwhen the demands of the drivetrain 114 are sufficiently high to requirea pressure within the hydraulic accumulator 116 to be in excess of apressure rating and/or a working pressure of the hydraulic accumulator116. The pressure rating and/or the working pressure of the hydraulicaccumulator 116 can correspondingly be lowered in a hydraulic systemthat can switch between a mode (e.g., the hydrostatic mode 86) where thehydraulic accumulator 116 is isolated and a mode (e.g., the hybridpropel mode 84) where the hydraulic accumulator 116 is connected. In thehydrostatic mode 86, the steering control unit 600 may receive powerfrom the prime mover 104.

The control system 500 may rapidly switch between the work circuitprimary mode 82, the hybrid propel mode 84, and/or the hydrostatic mode86 to continuously adjust the hydraulic system 100 to the demands of thework machine 50.

Turning now to FIG. 1, the hydraulic system 100 is illustrated as aschematic diagram. The hydraulic system 100 is powered by the primemover 104 which is connected to a pump/motor 102. In certainembodiments, the pump/motor 102 may be replaced with a pump. Asdepicted, the hydraulic system 100 allows the hydraulic pump/motor 102to be a single pump/motor (or a single pump) that powers the drivetrain114, the work circuit 300, and/or the steering control unit 600. Byconfiguring the hydraulic system 100 with the single pump/motor (or thesingle pump), a cost of the hydraulic system 100 may be reduced, aweight of the hydraulic system 100 may be reduced, the efficiency of thehydraulic system 100 may be increased by reducing the parasitic lossesof additional components, and/or a packaging size of the hydraulicsystem 100 may be reduced.

As depicted, the hydraulic pump/motor 102 and the prime mover 104 may beassembled into an engine pump assembly 106. In certain embodiments, theprime mover 104 turns in a single rotational direction (e.g., aclockwise direction), and thus, the hydraulic pump/motor 102 may alsorotate in the single rotational direction of the prime mover 104. Powermay be transferred between the hydraulic pump/motor 102 and the primemover 104 by a shaft (e.g., an input/output shaft of the hydraulicpump/motor 102 may be connected to a crankshaft of the prime mover 104).The power is typically transferred from the prime mover 104 to thehydraulic pump/motor 102 when the hydraulic pump/motor 102 is supplyinghydraulic power to the hydraulic accumulator 116, the drivetrain 114,the work circuit 300, and/or the steering control unit 600. The powermay be transferred from the hydraulic pump/motor 102 to the prime mover104 when the hydraulic pump/motor 102 is starting the prime mover 104,during engine braking, etc.

The hydraulic pump/motor 102 may be a variable displacement pump/motor.The hydraulic pump/motor 102 may be an over-center pump/motor. Thehydraulic pump/motor 102 includes an inlet 1021 (i.e., a low pressureside) that receives hydraulic fluid from a tank 118 via a low pressureline 440, and the hydraulic pump/motor 102 includes an outlet 102 h(i.e., a high pressure side) that is connected to a high pressure line400 of the hydraulic pump/motor 102. When the prime mover 104 suppliespower to the hydraulic pump/motor 102, hydraulic fluid is drawn from thetank 118 into the inlet 1021 of the hydraulic pump/motor 102 andexpelled from the outlet 102 h of the hydraulic pump/motor 102 at ahigher pressure. In certain embodiments, power may be delivered from thehydraulic pump/motor 102 to the prime mover 104 when a swash plate ofthe hydraulic pump/motor 102 is positioned over center and high pressurehydraulic fluid from the high pressure line 400 is driven backwardsthrough the hydraulic pump/motor 102 and ejected to the low pressureline 440 and to the tank 118. Alternatively, as illustrated at FIG. 8, areversing valve 103 of a hydraulic system 100′ can be used to cause theprime mover 104 to be backdriven with a hydraulic pump/motor 102′,similar to the hydraulic pump/motor 102.

A flow control device 202 (e.g., a relief valve) includes a connectionto the high pressure line 400. Upon hydraulic fluid pressure within thehigh pressure line 400 reaching a predetermined limit, the flow controldevice 202 opens and dumps a portion of the hydraulic fluid to the tank118 and thereby protecting the high pressure line 400 from reaching anover pressure condition.

A flow control device 206 is connected between the high pressure line400 and a high pressure line 406 of the work circuit 300. In thedepicted embodiment, the flow control device 206 is a work circuitvalve.

A flow control device 208 is connected between the high pressure line400 and a high pressure line 402. As depicted, the high pressure line402 may be connected to an inlet 108 h (i.e., a high pressure side) of apump/motor 108. The flow control device 208 may be an isolation valve.In certain embodiments, the flow control device 206 and the flow controldevice 208 may be combined into a single three-way valve 207 (see FIG.8).

The high pressure line 402 is connected to the hydraulic accumulator 116by a fluid flow control device 210. In the depicted embodiment, thefluid flow control device 210 is an isolation valve for the hydraulicaccumulator 116. In the depicted embodiment, the fluid flow controldevice 210 and the hydraulic accumulator 116 are connected by anaccumulator line 404.

The high pressure line 402 is further connected to the high pressureline 406 by a flow control device 212 and another flow control device224. In the depicted embodiment, the flow control device 212 is aValvistor® proportional flow control device, and the flow control device224 is a check valve that prevents hydraulic fluid from the highpressure line 406 from entering the high pressure line 402. In thedepicted embodiment, the flow control devices 212 and 224 are connectedin series along a cross-over flow line 408 that connects the highpressure line 402 and the high pressure line 406. In other embodiments,a single flow control device may be used along the cross-over flow line408.

Certain aspects of the propulsion system of the work machine 50 will nowbe described. The propulsion system includes the pump/motor 108 thatboth transmits and receives power to and from the drivetrain 114 via anoutput shaft 110. In particular, the output shaft 110 is connected to agear box 112. As illustrated at FIG. 9, the gear box 112 may include adifferential connected to a pair of the drive wheels 54. In otherembodiments, a hydraulic pump/motor may be included at each of the drivewheels 54, and the differential may not be used. When sending power tothe drivetrain 114, the pump/motor 108 may accelerate the work machine50, may move the work machine 50 up an incline, and/or may otherwiseprovide overall movement to the work machine 50. When the work machine50 decelerates and/or travels down an incline, the pump/motor 108 mayreceive energy from the drivetrain 114. When the hydraulic system 100 isin the hybrid propel mode 84 or the work circuit primary mode 82, thepump/motor 108 may send hydraulic energy to the hydraulic accumulator116. In particular, the pump/motor 108 may receive hydraulic fluid fromthe tank 118 via the low pressure line 440 and pressurize the hydraulicfluid and send it through the high pressure line 402 through the fluidflow control device 210 and the accumulator line 404 and into thehydraulic accumulator 116.

The pump/motor 108 may be driven by hydraulic power from the hydraulicaccumulator 116 or the hydraulic pump/motor 102. In particular, when thehydraulic system 100 is in the work circuit primary mode 82, thepump/motor 108 receives the hydraulic power from the hydraulicaccumulator 116, as illustrated at FIG. 3. When the hydraulic system 100is in the hybrid propel mode 84, as illustrated at FIGS. 4 and 5, thepump/motor 108 may receive hydraulic power from either the hydraulicpump/motor 102, the hydraulic accumulator 116, or both the hydraulicpump/motor 102 and the hydraulic accumulator 116. When the hydraulicsystem 100 is in the hydrostatic mode 86, as illustrated at FIGS. 6 and7, the pump/motor 108 receives power from the hydraulic pump/motor 102.However, the pump/motor 108 may deliver power to the hydraulicpump/motor 102 and the prime mover 104 may thereby provide enginebraking.

A relief valve 214 may be connected between the high pressure line 402and the tank 118. Feedback from the high pressure line 402 may be givento the hydraulic pump/motor 102 by way of a pump/motor control pressurevalve 220 (e.g. a pressure reducing valve). In particular, a point ofuse filter device 222 is connected between the high pressure line 402and the pump/motor control pressure valve 220. The pump/motor controlpressure valve 220 may feed a pressure signal to the hydraulicpump/motor 102 and thereby control the hydraulic pump/motor 102 incertain embodiments and/or in certain modes.

In the depicted embodiment, the steering control unit 600 receiveshydraulic power from the high pressure line 402. In particular, anintermediate pressure steering line 420 is connected to the highpressure line 402 via a steering feed valve 218 (e.g., a flow controlvalve) and a steering feed valve 216 (e.g., a pressure reducing valve).A return line 422 is connected between the steering control unit 600 andthe tank 118.

Various components may be included in a manifold block 200. For example,the flow control device 202, the flow control device 206, the flowcontrol device 208, the fluid flow control device 210, the flow controldevice 212, the relief valve 214, the pump/motor control pressure valve220, the device 222, and/or the flow control device 224 may be includedin the manifold block 200.

Turning now to FIG. 2, a schematic diagram of the control system 500 isshown with a schematic diagram of the hydraulic system 100. As can beseen, the hydraulic system 100 monitors a plurality of sensorsindicating the state of the hydraulic system 100. The control system 500further monitors the operator interface 506 thereby allowing an operatorto take control of the hydraulic system 100 and thereby take control ofthe work machine 50. The electronic control unit 502 of the controlsystem 500 may perform calculations that model the hydraulic system 100in the various modes and thereby determine the optimal mode and therebyselect the optimal mode for the given working conditions and the givenoperator input. Under certain conditions, the mode of the hydraulicsystem 100 is selected to maximize fuel efficiency of the work machine50. In other conditions, the mode of the hydraulic system 100 isselected to maximize performance of the hydraulic system 100 and therebythe work machine 50. The electronic control unit 502 may learn a workingcycle that the work machine 50 repeatedly undertakes. By learning theworking cycle, the electronic control unit 502 can maximize efficiencyfor the working cycle and identify when the work machine 50 is in theworking cycle. The electronic control unit 502 may switch modesdifferently depending on which working cycle the work machine 50 is in.By switching modes throughout the working cycle, various parameters ofthe hydraulic system 100 can be optimized for efficiency or performance.For example, charge pressure of the hydraulic accumulator 116, swashplate angle of the hydraulic pump/motor 102 and/or the pump/motor 108,and/or the timing of starting and stopping the prime mover 104 may bedetermined based on the working cycle of the work machine 50. Thecontrol system 500 may emulate a conventional work machine such that thework machine 50 behaves and feels like the conventional work machine tothe operator.

Turning now to FIG. 3, the work circuit primary mode 82 is illustrated.The work circuit primary mode 82 is selected by the control system 500when the work attachment 52 is under heavy use, sustained use, and/oruse that requires high volumetric flow rates of hydraulic fluid. Thedrivetrain 114 of the work machine 50 is operational in the work circuitprimary mode 82. In particular, the hydraulic accumulator 116 can supplypower to and receive power from the pump/motor 108. Upon the hydraulicaccumulator 116 being depleted to a given level, the control system 500may quickly switch the hydraulic system 100 into the hybrid propel mode84 to recharge the hydraulic accumulator 116. Upon the hydraulicaccumulator 116 being recharged to a given pressure level, the controlsystem 500 may return the hydraulic system 100 to the work circuitprimary mode 82.

Turning now to FIG. 4, the hybrid propel mode 84 is illustrated. Inparticular, a hybrid mode 84 a is illustrated. The hybrid mode 84 aallows the exchange of energy between the hydraulic pump/motor 102, thehydraulic accumulator 116, and the pump/motor 108. In particular, thehydraulic pump/motor 102 may supply hydraulic power to the hydraulicaccumulator 116 for the purpose of recharging the hydraulic accumulator116. The hydraulic pump/motor 102 may separately or simultaneouslysupply power to the pump/motor 108 to propel the work machine 50. Thehydraulic accumulator 116 may supply power to the hydraulic pump/motor102 for the purpose of starting the prime mover 104. Separately orsimultaneously, the hydraulic accumulator 116 may supply power to thepump/motor 108 to propel the work machine 50. The pump/motor 108 maysupply hydraulic fluid power to the hydraulic accumulator 116 andthereby charge the hydraulic accumulator 116. Separately orsimultaneously, the pump/motor 108 may provide power to the hydraulicpump/motor 102. The power supply to the hydraulic pump/motor 102 can beused to start the prime mover 104 and/or to provide engine braking(e.g., upon the hydraulic accumulator 116 being full). When thehydraulic system 100 is in the hybrid mode 84 a, the work circuit 300may be cut off from hydraulic fluid power. In this case, the workcircuit 300 may have no demand for hydraulic power.

Turning now to FIG. 5, the hybrid propel mode 84 is again illustrated.In particular, a hybrid mode 84 b is illustrated. The hybrid mode 84 bis similar to the hybrid mode 84 a except that the cross-over flow line408 is open allowing hydraulic fluid power from the high pressure line402 to be supplied to the work circuit 300. In the hybrid mode 84 b, thehydraulic pump/motor 102, the hydraulic accumulator 116, and/or thepump/motor 108 may supply hydraulic power to the work circuit 300.

The hybrid propel mode 84 may be preferred when the work machine 50 isundergoing a moderate workload, and/or when high efficiency and/orenergy recovery from the drivetrain 114 is desired.

Turning now to FIG. 6, the hydrostatic mode 86 is illustrated. Inparticular a hydrostatic mode 86 a is illustrated. The hydrostatic mode86 a may be used when the drivetrain 114 of the work machine 50 is underheavy load. For example, when the work machine 50 is driven at hightorque/power and/or when the work machine 50 is driven up an incline.When the hydraulic system 100 is operated in the hydrostatic mode 86 a,hydraulic pressure within the high pressure line 400 and the highpressure line 402 may exceed a working pressure and/or a rated pressureof the hydraulic accumulator 116. By switching between the hybrid propelmode 84 and the hydrostatic mode 86, the hydraulic system 100 mayundertake tasks that result in high pressures in the high pressure line402 without exposing the hydraulic accumulator 116 to the highpressures. Thus, the benefits of the hybrid propel mode 84 can beenjoyed without requiring that the accumulator 116 have a pressurerating that matches the maximum pressure rating of the hydraulicpump/motor 102. By bypassing (e.g., isolating) the accumulator 116 withthe fluid flow control device 210, the hydraulic system 100 does notneed to wait for the accumulator 116 to be pressurized up to the desiredworking pressure. When the hydraulic system 100 is in the hydrostaticmode 86 a, the work circuit 300 may be cut off from hydraulic fluidpower. In this case, the work circuit 300 may have no demand forhydraulic power.

Turning now to FIG. 7, the hydrostatic mode 86 is further illustrated.In particular, a hydrostatic mode 86 b is illustrated. The hydrostaticmode 86 b is similar to the hydrostatic mode 86 a, except that thecross-over flow line 408 is open allowing hydraulic fluid power from thehigh pressure line 402 to be supplied to the work circuit 300. In thehydrostatic mode 86 b, the hydraulic pump/motor 102 and/or thepump/motor 108 may supply hydraulic power to the work circuit 300.

Turning now to FIG. 8, a system forming a second embodiment of theprinciples of the present disclosure is presented. The system includesthe hydraulic system 100′, mentioned above. As many of the concepts andfeatures are similar to the first embodiment, shown at FIGS. 1-7, thedescription for the first embodiment is hereby incorporated by referencefor the second embodiment. Where like or similar features or elementsare shown, the same reference numbers will be used where possible. Thefollowing description for the second embodiment will be limitedprimarily to the differences between the first and second embodiments.In the hydraulic system 100′, the flow control device 206 and the flowcontrol device 208 of the hydraulic system 100 have been replaced by thesingle three-way valve 207. In addition, the flow control device 212 andthe flow control device 224 of the hydraulic system 100 has beenreplaced by an on-off electrically controlled valve 212′ and a constantflow valve 224′. The substitution of the on-off electrically controlledvalve 212′ and the constant flow valve 224′ can be further made in otherembodiments of the present disclosure. Likewise, the flow control device212 and the flow control device 224 can be substituted in the presentembodiment.

Turning now to FIG. 9, a schematic layout of the work machine 50 isillustrated. In the depicted embodiment, the work machine 50 is a forktruck.

Turning now to FIG. 10, a system forming a third embodiment of theprinciples of the present disclosure is schematically illustrated. Thesystem includes a hydraulic system 100″. As with the hydraulic system100, the hydraulic system 100″ similarly powers the work circuit 300.However, in the hydraulic system 100″ a hydraulic pump 107 is used toprovide hydraulic power to the work circuit 300. The hydraulic pump 107,in turn, is connected by a shaft 109 to a pump/motor 102″. A clutch 105is operably connected between the prime mover 104 and the hydraulicpump/motor 102″. A low pressure accumulator 117 (i.e., a storageaccumulator) is further included connected to a low pressure side of thehydraulic pump/motor 102″.

By placing the hydraulic pump/motor 102″ at a zero swash platedisplacement angle, power can flow from the prime mover 104 through theclutch 105 and into the hydraulic pump 107. Thus, power from the primemover 104 can directly power the work circuit 300. While the prime mover104 is directly powering the work circuit 300, the hydraulic accumulator116 can be both supplying and receiving power from the pump/motor 108.Thus, the hydraulic system 100″ has a mode similar to the work circuitprimary mode 82, illustrated at FIG. 3.

Hydraulic power from the hydraulic accumulator 116 can be used to startthe prime mover 104. In particular, hydraulic power flows from thehydraulic accumulator 116, through fluid flow control device 210, andinto the hydraulic pump/motor 102″. The clutch 105 can be engaged andthereby the hydraulic pump/motor 102″ can start the prime mover 104.

The hydraulic pump/motor 102″, the hydraulic accumulator 116, thepump/motor 108, and the prime mover 104 can operate in a hybrid propelmode similar to the hybrid propel mode 84. When hydraulic power isrequired by the work circuit 300, the hydraulic pump 107 can receivepower from the hydraulic pump/motor 102″ via the shaft 109. Thus, thehydraulic system 100″ has a mode similar to the hybrid mode 84 b,illustrated at FIG. 5.

The hydraulic accumulator 116 can be isolated from the pump/motor 108 byclosing the fluid flow control device 210. In this way, the hydraulicsystem 100″ can operate in a hydrostatic mode similar to the hydrostaticmode 86. If the work circuit 300 requires hydraulic power, the hydraulicpump 107 may receive power from the hydraulic pump/motor 102″ via theshaft 109.

According to the principles of the present disclosure, an examplealgorithm may be incorporated in the control of the hydraulic system100. The example algorithm includes nine major components.

The first major component of the example algorithm is selecting thehydrostatic mode (e.g., the hydrostatic mode 86) with the transmissionmode supervisor when the following conditions are met.

-   -   Current mode is hybrid mode AND    -   Pressure target is greater than a specific calibration AND    -   Pressure target is greater than the high pressure accumulator        pressure AND    -   Accelerator pedal command (e.g., percentage) is greater than a        calibrated value (e.g., 50%) AND    -   Hydrostatic mode is enabled AND    -   Work circuit flow demand is zero AND    -   Vehicle speed is less than a specific calibration (e.g., 7 MPH)        AND    -   Vehicle is hot shifted at low specific calibrated speed OR        vehicle is not hot shifted

The second major component of the example algorithm includes: a)commanding the drive motor displacement target to 100% (or some otherpredetermined value) and b) changing the engine pump mode from hybridmode to a transition mode between the hybrid and hydrostatic modes withthe engine and pump supervisory process. In particular, the engine andpump supervisory process (i.e., the engine supervisor) calculates threecritical targets: a) engine state (on/off), b) engine speed target, andc) engine pump displacement target. The following summarizes thecalculations used to calculate these values.

-   -   The engine state target is changed to the “on” state in        hydrostatic mode.    -   The engine power target        _(target) is calculated from the fundamental equation:        _(target) =P _(target) Q _(reqd) where:    -   P_(target) is the pressure target calculated from operator        commands    -   Q_(reqd) is the engine pump flow required to achieve the        pressure target        -   Q_(reqd) is calculated using:            Q _(reqd) =Q _(dm) +Q _(wc) +Q _(leak) +Q _(ep-target)            -   Where:            -   Q_(dm) is the existing drive motor flow consumption            -   Q_(wc) is the existing work circuit flow consumption            -   Q_(leak) is the existing system leakage            -   Q_(ep-tgt) is the additional engine pump flow required                into the manifold to achieve the target pressure            -   Q_(dm) is calculated using:                Q _(dm)=ω_(dm) D _(max-dm) x _(dm)                -   Where:                -   ω_(dm) is the sensed drive motor speed                -   D_(max-dm) is the theoretical maximum drive motor                    displacement                -   x_(dm) is the sensed fraction of maximum drive motor                    displacement            -   Q_(wc) is calculated using:                Q _(wc) =Q _(lift-dmd) +Q _(tilt-dmd) +Q _(shift-dmd)                -   Where:                -   Q_(lift-dmd) is the lift flow demand                -   Q_(tilt-dmd) is the tilt flow demand                -   Q_(shift-dmd) is the shift flow demand                -   The flow demand for each service is calculated                    using:                    Q _(x-dmd) =dmr _(x) A _(x-cyl)                -   Where:                -   dmr_(x) is the driver mast request (dmr) speed for                    service “x”                -   The “dmr” for each service is a calibrated look up                    table of target cylinder speed vs. operator lever                    command.                -   A_(x-cyl) is the cross-sectional area of the                    cylinder for service “x”            -   Q_(leak) is estimated from a transfer function based on                sensitive factors such as drive motor speed, engine pump                speed, and system pressure.            -   Q_(ep-tgt) is calculated from:

$Q_{{ep} - {tgt}} = \frac{V_{{fl} - {cur}} - V_{{fl} - {tgt}}}{{DT}_{tgt}}$

-   -   -   -   The basis for this equation is to calculate how much                additional fluid needs to be pumped into the manifold to                achieve the target system pressure.                -   Where:                -   V_(fl-cur) is the current volume of fluid in the                    system manifold                -   V_(fl-cur) is calculated from:

$V_{{fl} - {cur}} = {\frac{P_{man}}{B/V_{man}} + V_{man}}$

-   -   -   -   -   The basis for this equation is that of a fixed                    manifold block pressure calculation solved for the                    current volume of fluid in the manifold                -    Where:                -    P_(man) is the sensed pressure in the system                    manifold                -    V_(man) is the volume of the system manifold cavity                -    B is the bulk modulus of the hydraulic system fluid                -   V_(fl-tgt) is the target volume of fluid in the                    system manifold                -   The basis for this equation is that of a fixed                    manifold block pressure calculation solved for the                    current volume of fluid in the manifold                -   V_(fl-tgt) is calculated from:

$V_{{fl} - {cur}} = {\frac{P_{target}}{B/V_{man}} + V_{man}}$

-   -   -   -   -    Where:                -    P_(target) is the target pressure as described                    earlier                -    V_(man) is the volume of the system manifold cavity                -    B is the bulk modulus of the hydraulic system fluid                -   DT_(tgt) is the target time to achieve the target                    volume of fluid in the system manifold.                -   This value is a calibration but will control the                    speed of the reaction to a change in pressure                    target. The smaller DT_(tgt) is, the faster the                    algorithm will react.

    -   The engine speed target ω_(eng-tgt) is calculated using an        appropriate method.

    -   The engine pump displacement target is calculated using:

$x_{ep} = {\frac{{\mathbb{P}}_{target}}{P_{man}D_{{{ma}\; x} - {ep}}\omega_{{eng} - {tgt}}}.}$

-   -   This equation is based on the fundamental equation to calculate        power from torque and speed.    -   Where:    -   _(target) is the engine power target (calculated above)    -   D_(max-ep) is the theoretical maximum engine pump displacement    -   ω_(eng-tgt) is the engine speed target (calculated above)    -   P_(man) is the sensed pressure in the system manifold under        normal hydrostatic mode operation. When the system is in        transition states the following pressure values are used:    -   1.) If the high pressure accumulator (hpa) isolation valve        target is to be open, then the pressure target should be used to        calculate pump displacement. This is to predict what the        pressure will be once the valve closes and prevent the pressure        from spiking once the hpa isolation valve is closed.    -   2.) Otherwise, if target isolation valve is closed, but the        transmission mode is still not hydrostatic, very likely there        will be a pressure spike appearing. Use the max hpa pressure        calibration value to limit what the system pressure will be and        this will keep pump displacement target from reducing too much        and hence prevent the pressure from cavitating once the initial        pressure spike is over.    -   If neither of the above is true and if the pressure in the        manifold has begun to cavitate and sensors are reporting a value        less than a small (calibration) value (e.g., 10 bar) replace        that small value with 10 bar by default to prevent divide by        zero errors when calculating the resulting pump displacement.        (see the equation above). This also keeps continuity for the        transition.

The third major component of the example algorithm includes configuringvalves to be compatible with the hydrostatic mode with the valvesupervisor. In particular:

-   -   i. The Main Isolation valve is commanded to be open    -   ii. The EP on valve is commanded to be shut    -   iii. The EP off valve is commanded to provide work circuit flow        as is required from the operator commands    -   iv. The valve supervisor closes the high pressure accumulator        isolation valve once the following conditions are true:        -   1. The selected transmission mode is hydrostatic        -   2. The drive motor pressure rate of change is greater than            zero or small value. (Calibration).            -   This is required because a positive drive motor pressure                rate of change indicates that the engine pump has                achieved equivalent or slightly higher flow output than                the system is consuming. If the high pressure                accumulator isolation valve commanded to be closed right                as the engine pump flow output matches the system flow                consumption, there will be flow continuity and the                manifold pressure will behave in a predictable manner                (this is the desired scenario). If flow matching is not                achieved before the valve is closed the vehicle will                either surge or drop in speed. Surging will occur when                the engine pump is providing significantly more flow                than the drive motor is consuming. If the engine pump is                providing less flow than the drive motor is consuming,                the drive motor will begin to cavitate and deceleration                will occur until pump flow is matched with motor flow.                Additionally, if pump control pressure is dependent upon                the developed hydrostatic line pressure, control                pressure will be lost and the drive motor and engine                pump will return to their default positions until                control pressure is restored.        -   3. Engine speed target is greater than a specified            calibration (larger than engine idle speed)        -   4. Engine speed is greater than a specified calibration.            (Should be same as calibration from previous)            -   The engine will stall at low speeds if the high pressure                accumulator isolation valve is closed. The stall is                caused by pressure spikes and slow engine pump                displacement response to the resulting torque caused by                the pressure spike. This high torque is greater than the                maximum engine torque at the particular speed and hence                the engine stalls. At higher speeds the engine is more                able to handle a transitory torque spike than at low                speeds due to rotational inertia, higher torque                capacity, and the larger recovery time allowed before an                engine stall would occur. The reason for the target and                actual engine speed requirements is that the system                should only close the valve when engine speed has been                requested and observed to be above this value. It                prevents bad calibrations and accidental shifts because                of engine speed oscillations.        -   5. Drive motor pressure is greater than the high pressure            accumulator pressure by a calibrated value (usually a small            negative value)            -   This requirement is to prevent the high pressure                accumulator isolation valve from closing (e.g.,                immediately closing) after opening. It requires that                normal hybrid pressure (based on the high pressure                accumulator pressure) is restored before re-attempting                to enter hydrostatic mode. This prevents rapid                oscillations of the high pressure accumulator isolation                valve during transitions into hydrostatic mode.    -   v. The valve supervisor opens the HPA isolation valve while the        system is in hydrostatic mode in the following events:        -   1. The drive motor pressure drops below a calibrated level            (e.g., 50 bar)            -   This is to prevent cavitation in the motor and a loss of                torque output.        -   2. The engine state status is sensed as “Off” (not ready to            pump)            -   This is to ensure the motor has a stable source of                hydraulic oil.

The fourth major component of the example algorithm includes determiningthat the current transmission mode is hydrostatic with the transmissionmode supervisor when the following conditions are met.

-   -   Low level state is “Accel” (i.e., operator intent indicates work        machine should accelerate) AND    -   The engine is confirmed to be “on” and “ready” for the engine        pump displacement to increase AND    -   The high pressure accumulator isolation valve is confirmed to be        closed AND    -   The ep on valve is confirmed to be closed AND    -   The main isolation valve is confirmed to be open AND    -   The drive motor displacement status is confirmed to be greater        than a specified value (e.g., calibration ˜90%) AND    -   The previous selected transmission mode is hydrostatic AND    -   hydrostatic mode is enabled

The fifth major component of the example algorithm includes changing theengine pump mode from the transition mode to the hydrostatic mode oncethe current transmission mode is hydrostatic. The engine behavior may bethe same as in the transition mode.

The sixth major component of the example algorithm includes exiting thehydrostatic mode with the transmission mode supervisor when theconditions below are met for a predetermined period of time.

-   -   The target pressure is less than the high pressure accumulator        pressure status OR    -   The accelerator pedal is less than a calibrated value OR    -   The low level state is “Decelerate” OR    -   The low level state is “Neutral” OR    -   The engine state is “Off” (i.e., not ready to pump)

The seventh major component of the example algorithm includes exitingthe hydrostatic mode immediately when a system fault is detected.

The eighth major component of the example algorithm includes exiting thehydrostatic mode with the engine pump when the transmission modesupervisor exits the hydrostatic mode and transitions to normal hybridmode.

The ninth major component of the example algorithm includes opening thehigh pressure accumulator isolation valve when the transmission modesupervisor exits HSTAT mode.

In certain embodiments, the functions or sets of functions describedabove may be accomplished with a single drive pump component (e.g., asingle pump, a single pump/motor, a single pumping rotating group,etc.). As used herein, the term “pump” indicates the ability to transferfluid from a lower pressure to a higher pressure over a durationsufficient to power a function. The single drive pump may include acharge pump. As used herein, the terms “drive pump” and “drive hydraulicpump” indicate a pump or pump/motor that is driven by the prime mover(e.g., directly mechanically driven).

The various embodiments described above are provided by way ofillustration only and should not be construed to limit the claimsattached hereto. Those skilled in the art will readily recognize variousmodifications and changes that may be made without following the exampleembodiments and applications illustrated and described herein, andwithout departing from the true spirit and scope of the disclosure.

What is claimed is:
 1. A method of configuring a propulsion mode of amobile work vehicle having a hydraulic circuit system including a highpressure line extending between an engine-pump assembly, a work circuit,and a propel circuit, the engine-pump assembly including a prime moverand a hydraulic pump/motor, the work circuit including an actuator forpowering a work component of the mobile work vehicle, the propel circuitincluding a drive motor for powering propulsion elements of the mobilework vehicle through a drivetrain, the hydraulic circuit systemincluding a hydraulic accumulator that is selectively coupled to thehigh pressure line via an accumulator isolation valve, the mobile workvehicle being configured to selectively operate in a hybrid propulsionmode and a hydrostatic propulsion mode, the method comprising:determining if a selected propulsion mode is the hydrostatic propulsionmode; substantially matching a flow output of the engine-pump assemblyto a system flow consumption if the selected propulsion mode is thehydrostatic propulsion mode; and closing the accumulator isolation valveto isolate the hydraulic accumulator from the high pressure line whenthe engine-pump flow output matches the system flow consumption and theselected propulsion mode is the hydrostatic propulsion mode.
 2. Themethod of claim 1, further comprising configuring a drive motordisplacement target of the drive motor to a pre-determined displacementif the selected propulsion mode is the hydrostatic propulsion mode. 3.The method of claim 2, wherein the pre-determined displacement is fulldisplacement.
 4. The method of claim 1, wherein the hybrid propulsionmode includes pressure based control and the hydrostatic propulsion modeincludes displacement based control.
 5. The method of claim 1, furthercomprising calculating the system flow consumption and therebymaintaining flow continuity when closing the accumulator isolationvalve.
 6. The method of claim 5, wherein the calculating of the systemflow consumption includes calculating a flow consumption of a workcircuit.
 7. The method of claim 5, wherein the calculating of the systemflow consumption includes calculating a flow consumption of a steeringcircuit.
 8. The method of claim 1, wherein determining if the selectedpropulsion mode is the hydrostatic propulsion mode includes evaluatingif an acceleration request of an operator requires a propulsion pressuretarget that is greater than an accumulator pressure of the hydraulicaccumulator.
 9. The method of claim 1, wherein determining if theselected propulsion mode is the hydrostatic propulsion mode includesdetermining if the hydrostatic propulsion mode is enabled.
 10. Themethod of claim 1, further comprising: setting an engine state target toan on state when the accumulator isolation valve is closed; setting anengine speed target to a calculated value when the accumulator isolationvalve is closed; and configuring a valve set in a configurationcompatible with the hydrostatic propulsion mode when the accumulatorisolation valve is closed.
 11. The method of claim 1, further comprisingopening the accumulator isolation valve when in the hydrostaticpropulsion mode and when a drive motor pressure drops below apredetermined level.
 12. The method of claim 1, wherein the accumulatorisolation valve is closed both when the engine-pump flow output matchesthe system flow consumption and a drive-motor pressure rate of change isgreater than a predetermined value.
 13. The method of claim 1, furthercomprising powering the hybrid propulsion mode and the hydrostaticpropulsion mode with a same pump.
 14. The method of claim 1, wherein thepropulsion elements of the work vehicle includes either wheels ortracks.
 15. The method of claim 1, further comprising using energyaccumulated in the hydraulic accumulator to provide power for starting apower source used to drive the hydraulic propulsion pump/motor.
 16. Themethod of claim 15, further comprising using energy accumulated in thehydraulic accumulator to provide propulsion functionality even when thepower source coupled to the hydraulic propulsion pump/motor is notoperating.
 17. The method of claim 15, further comprising using energyaccumulated in the hydraulic accumulator to provide work circuitfunctionality.
 18. A method of configuring a propulsion mode of a mobilework vehicle having a hydraulic circuit system including a high pressureline extending between an engine-pump assembly, a hydraulic accumulator,and a drive motor for powering propulsion elements of the mobile workvehicle through a drivetrain, the hydraulic circuit system including anaccumulator isolation valve that selectively couples the hydraulicaccumulator to the high pressure line, the mobile work vehicle beingconfigured to selectively operate in a hybrid propulsion mode and ahydrostatic propulsion mode, the method comprising: determining if aselected propulsion mode is the hydrostatic propulsion mode;substantially matching a flow output of the engine-pump assembly to asystem flow consumption if the selected propulsion mode is thehydrostatic propulsion mode; and closing the accumulator isolation valveto isolate the hydraulic accumulator from the high pressure line whenthe engine-pump flow output matches the system flow consumption and theselected propulsion mode is the hydrostatic propulsion mode; configuringa drive motor displacement target of the drive motor to a pre-determineddisplacement if the selected propulsion mode is the hydrostaticpropulsion mode, wherein the pre-determined displacement is fulldisplacement.
 19. A method of configuring a propulsion mode of a mobilework vehicle having a hydraulic circuit system including a high pressureline extending between an engine-pump assembly, a hydraulic accumulator,and a drive motor for powering propulsion elements of the mobile workvehicle through a drivetrain, the hydraulic circuit system including anaccumulator isolation valve that selectively couples the hydraulicaccumulator to the high pressure line, the mobile work vehicle beingconfigured to selectively operate in a hybrid propulsion mode and ahydrostatic propulsion mode, wherein the hybrid propulsion mode includespressure based control and the hydrostatic propulsion mode includesdisplacement based control, the method comprising: determining if aselected propulsion mode is the hydrostatic propulsion mode;substantially matching a flow output of the engine-pump assembly to asystem flow consumption if the selected propulsion mode is thehydrostatic propulsion mode; and closing the accumulator isolation valveto isolate the hydraulic accumulator from the high pressure line whenthe engine-pump flow output matches the system flow consumption and theselected propulsion mode is the hydrostatic propulsion mode.
 20. Amethod of configuring a propulsion mode of a mobile work vehicle havinga hydraulic circuit system including a high pressure line extendingbetween an engine-pump assembly, a hydraulic accumulator, and a drivemotor for powering propulsion elements of the mobile work vehiclethrough a drivetrain, the hydraulic circuit system including anaccumulator isolation valve that selectively couples the hydraulicaccumulator to the high pressure line, the mobile work vehicle beingconfigured to selectively operate in a hybrid propulsion mode and ahydrostatic propulsion mode, the method comprising: determining if aselected propulsion mode is the hydrostatic propulsion mode, whereindetermining if the selected propulsion mode is the hydrostaticpropulsion mode includes evaluating if an acceleration request of anoperator requires a propulsion pressure target that is greater than anaccumulator pressure; substantially matching a flow output of theengine-pump assembly to a system flow consumption if the selectedpropulsion mode is the hydrostatic propulsion mode; and closing theaccumulator isolation valve to isolate the hydraulic accumulator fromthe high pressure line when the engine-pump flow output matches thesystem flow consumption and the selected propulsion mode is thehydrostatic propulsion mode.
 21. A method of configuring a propulsionmode of a mobile work vehicle having a hydraulic circuit systemincluding a high pressure line extending between an engine-pumpassembly, a hydraulic accumulator, and a drive motor for poweringpropulsion elements of the mobile work vehicle through a drivetrain, thehydraulic circuit system including an accumulator isolation valve thatselectively couples the hydraulic accumulator to the high pressure line,the mobile work vehicle being configured to selectively operate in ahybrid propulsion mode and a hydrostatic propulsion mode, the methodcomprising: determining if a selected propulsion mode is the hydrostaticpropulsion mode, wherein determining if the selected propulsion mode isthe hydrostatic propulsion mode includes determining if the hydrostaticpropulsion mode is enabled; substantially matching a flow output of theengine-pump assembly to a system flow consumption if the selectedpropulsion mode is the hydrostatic propulsion mode; and closing theaccumulator isolation valve to isolate the hydraulic accumulator fromthe high pressure line when the engine-pump flow output matches thesystem flow consumption and the selected propulsion mode is thehydrostatic propulsion mode.
 22. A method of configuring a propulsionmode of a mobile work vehicle having a hydraulic circuit systemincluding a high pressure line extending between an engine-pumpassembly, a hydraulic accumulator, and a drive motor for poweringpropulsion elements of the mobile work vehicle through a drivetrain, thehydraulic circuit system including an accumulator isolation valve thatselectively couples the hydraulic accumulator to the high pressure line,the mobile work vehicle being configured to selectively operate in ahybrid propulsion mode and a hydrostatic propulsion mode, the methodcomprising: determining if a selected propulsion mode is the hydrostaticpropulsion mode; substantially matching a flow output of the engine-pumpassembly to a system flow consumption if the selected propulsion mode isthe hydrostatic propulsion mode; and closing the accumulator isolationvalve to isolate the hydraulic accumulator from the high pressure linewhen the engine-pump flow output matches the system flow consumption andthe selected propulsion mode is the hydrostatic propulsion mode, whereinthe accumulator isolation valve is closed both when the engine-pump flowoutput matches the system flow consumption and a drive-motor pressurerate of change is greater than a predetermined value.